As is known, high-voltage resistors integrated on a semiconductor material substrate are used extensively in the field of integrated monolithic power devices, for example devices manufactured using VIPower technology, according to which the power devices are integrated in a first chip region, known as the power region, whereas the corresponding control devices are integrated in a second chip region, which is known as the control region, and is separated and electrically isolated from the power region.
In addition, in some applications, it is also necessary to have available within the control region a biasing voltage which is branched from the biasing voltage of the substrate, by means of a partition provided using a resistor connected between the substrate and the control region.
However, in order for this resistor to be able to withstand the high values (up to 2 kV) which, as is known, the biasing voltage of the substrate can reach, it must have somewhat high resistance values which generally vary between 100 kΩ and a few MΩ.
A solution which is commonly used to manufacture a resistor having the above-described resistance values consists of forming on the semiconductor substrate a doped region with high resistivity and having conductivity opposite that of the semiconductor substrate, and a flat coil pattern.
Although it is advantageous in various respects, this solution has the disadvantage that it requires a somewhat large surface area, owing mainly to the fact that, in order to prevent malfunctioning of the resistor, the minimum distance which must be maintained between two adjacent parallel branches of the coil resistor cannot be reduced as required, but depends on the concentration of doping agent present in the substrate, and on the voltage across the resistor.
In fact, as is known, when the junction formed by the substrate and the resistor is biased inversely, the size of the depletion or space-charge region which consequently extends in the substrate, is inversely proportional to the concentration of doping agent in the substrate, i.e., it is directly proportional to the resistivity of the substrate.
Consequently, although the high-voltage resistor can be integrated using the most resistive layers available in the technique, devices manufactured using VIPower technology and able to withstand high voltages, have necessarily high resistivity in the substrate, of several orders of magnitude greater than the most resistive layers available according to the present technological processes, and thus, the size of the depletion region extending in the substrate has somewhat large dimensions, of approximately tens of microns, when high differences of potential are applied.
From the foregoing, it is apparent that, in order to prevent the depletion regions of two adjacent parallel branches of the coil resistor from coming into contact, and giving rise to the known pinch-off phenomenon, thus giving rise to deterioration of the resistance value of the resistor, and therefore of the functionality of the circuitry to which this resistor is connected, during the design stage it is necessary to space each pair of adjacent parallel branches of the coil resistor, by a value which is greater than the sum of the maximum widths of the depletion region applicable for each branch.
In order to reduce the depletion region present between the various branches, a known solution consists of enriching the layer designed for integration of the resistor. However, this solution reduces the breakdown voltage of the device, since, in order to be able to obtain the required reduction of the depletion region, it would be necessary to have an extremely high concentration of doping agent.
The aforementioned large surface area of the coil resistor is also caused secondarily by the fact that the presence of high voltages on the resistor requires the formation of so-called edge structures which can protect against phenomena of premature breakdown of the regions of the resistor subjected most to the high voltages. In fact, for example, for this purpose, so-called metal field plates are formed, i.e., annular regions with high resistivity (low concentration of doping agent) and surrounding the coil resistor.
A further effect which contributes towards making the surface area of resistors of the above-described type large, is their interaction with the edge structures of the devices in which they are used, and the consequent necessity to arrange this resistor in the vicinity of the terminal region of the device from which the high voltage is obtained.
In order to reduce the depletion region present between the various branches of the coil resistor, a solution proposed recently, which is the subject of European Patent application 98830638.7 filed on Oct. 23, 1998, by the same applicant, consists of forming the coil resistor using a semiconductor material layer with high resistivity and having conductivity opposite that of the substrate, and, between each pair of adjacent parallel branches of the coil resistor, forming one or more isolation trenches, for example formed of silicon dioxide, extending in depth further into the substrate than the semiconductor material layer from which the coil resistor is formed, by an extent sufficient to prevent the pinch-off phenomenon from occurring.
However, also in this solution, the coil resistor is arranged close to the terminal region of the device from which the high voltage is obtained, and consequently the reduction of the surface area is relatively small, and there still exists the disadvantage caused by the interaction of the resistor with the edge structures of the device in which this resistor is formed.